Investigate Jekyll build improvements

[JackGilmore]

Is your feature request related to a problem? Please describe. Jekyll builds currently take about a minute on a good PC at the moment. We should see if we can cut this down. Output stats from a build:

Filename	Occurrences in build	Bytes	Time (s)
analytics_la_categories.html	1	83.74K	20.443
_layouts/dataset.html	1697	51051.82K	14.421
_includes/dataset-form.html	1698	39005.19K	11.584
_includes/form/organization.html	1698	18805.58K	4.669
_includes/dataset-form-resource.html	6563	8338.45K	3.031
_layouts/default.html	1910	71050.41K	2.969
_includes/head.html	1910	7584.38K	1.563
_includes/header.html	1910	7575.97K	1.219
data.json	1	2825.11K	1.05
sitemap.xml	1	291.94K	1.022
_includes/form/text.html	27044	6079.75K	0.936
_includes/form/dropdown.html	6758	3398.73K	0.849
_includes/breadcrumbs.html	1909	477.27K	0.562
_layouts/organization.html	194	1399.37K	0.562
_includes/form/category.html	1698	1719.94K	0.528
_includes/form/license.html	1698	3323.18K	0.437
_includes/organization-form.html	195	888.75K	0.435
datasets.json	1	2714.95K	0.305
organizations.html	1	163.61K	0.235
_includes/addons/twittercard.html	1910	1088.24K	0.198
_includes/display/category.html	1697	424.59K	0.178
_includes/addons/opengraph.html	1910	882.33K	0.177
_includes/form/org_type.html	195	219.29K	0.068
_includes/form/textarea.html	1698	1256.97K	0.062
_includes/display/link.html	1697	495.88K	0.048
admin.html	1	41.77K	0.038
analytics_platform_health.html	1	286.88K	0.037
analytics_la_coverage.html	1	4.04K	0.036
data/local_authorities.json	1	16.42K	0.036
organizations.json	1	89.52K	0.021
_includes/admin-form-category.html	18	21.03K	0.016
_includes/footer.html	1910	2100.25K	0.013
add-dataset.html	1	18.76K	0.006
add-organization.html	1	4.82K	0.002
_includes/admin-form-license.html	14	10.05K	0.002
index.html	1	6.29K	0.002
_includes/form/checkbox.html	18	3.01K	0.001
analytics_portal_types.html	1	2.20K	0
about_opendata.html	1	4.14K	0
analytics_file_types.html	1	2.53K	0
analytics_licensing.html	1	2.37K	0
about_siteanalytics.html	1	0.89K	0
resources.html	1	12.76K	0
jekyll-redirect-from-0.16.0/lib/jekyll-redirect-from/redirect.html	3	1.70K	0
datasets.html	1	0.96K	0
soocon23.html	1	1.72K	0
suggest_dataset.html	1	1.74K	0
about_project.html	1	4.41K	0
analytics.html	1	4.85K	0

Done in 65.089 seconds.

Describe the solution you'd like Investigate if build can be optimised with any of the techniques in this article: https://forestry.io/blog/how-i-reduced-my-jekyll-build-time-by-61/

Describe alternatives you've considered N/A

Additional context N/A